Harnessing the cytopathic effects of virus infection is rapidly becoming established as an anti-cancer treatment and replication-competent mutants of the alpha-herpesvirus, herpes simplex virus type 1 (HSV-1), are showing great potential as oncolytic therapeutic agents.1, 2 The HSV-1 mutant 1716, lacking the ICP34.5 gene, has greatly reduced lethality in mice but replicates as efficiently as wild-type virus in actively dividing tissue culture cells.3, 4 The ICP34.5 ORF is a neurovirulence gene and its protein product has been proposed to condition post-mitotic cells for viral replication, probably via an interaction with proliferating cell nuclear antigen.5, 6 ICP34.5 deletion mutants cannot replicate in terminally differentiated cells but will lytically infect dividing cells and this has proved to be an effective tumour destruction strategy. In recent clinical trials, injection of HSV1716 has been shown to be safe in treating patients with recurrent glioma, metastatic melanoma and squamous cell carcinoma of the head and neck (Mace et al unpublished results) and proof of principle of selective replication within tumours has been obtained.7-10 
Initiation of infection by HSV-1 requires cells to display the appropriate receptors to permit viral access, a process requiring the complex interplay of a number of cellular and viral membrane components.37, 38 Four virus membrane glycoproteins, gB, gD and the heterodimer comprising gH and gL (gH/gL) have been shown to be necessary and sufficient for HSV-1 entry into cells. Initial contact is between gB and cellular heparan sulphate, gD then interacts specifically with the cellular receptors for HSV-1 entry which include herpesvirus entry mediator (HVEM), nectins-1 and -2 and 3-O-sulphated heparan sulphate. Membrane fusion requires the concerted activities of gB and gH/gL so that the nucleocapsid gains access to the cell and infection is initiated. Nectin-1 is probably the principal entry receptor for infection of central and peripheral nerve cells whereas HVEM expression is more restricted and limited to cells of lymphoid origin.39-43 
An essential strategy for the improved effectiveness of oncolytic viral therapies depends upon systemic delivery of targeted viruses that seek out and destroy all cancerous cells.2, 11, 12 However, wide bioavailability of HSV-1 entry mediators will hinder the ability of systemically administered oncolytic HSV1716 to efficiently target the cells of interest.
Reprogramming viral tropisms has thus received much attention, an approach requiring redirection of the natural tropism from native receptors to a receptor of choice. Targeted infection by vaccinia virus, retrovirus and measles virus, displaying single chain antibody binding sites incorporated into their structure by fusion with viral envelope proteins has been described.13-21, 55 Bridging, bispecific targeting molecules comprising either an anti-adenovirus single chain antibody or the Coxsackievirus-adenovirus receptor linked to a targeting single chain antibody, have been used for targeting oncolytic adenoviruses.22-28 
Results from a number of studies with HSV-1 have shown that it is possible to alter the tropism by incorporating ligands such as erythropoeitin, IL13, human hepatitis B virus preS1 peptide, the N-terminal fragment of urokinase-type plasminogen activator or 6-His into the viral envelope as glycoprotein fusion proteins.29-34, 56, 57 
Targeting has also been achieved using a soluble adaptor molecule, in which the soluble adaptor molecule includes an EGFR scFv linked to the HSV-1 gD-binding domain of nectin-1. More recently, a scFv against HER2/neu, inserted within an N terminal region of gD, has been shown to redirect the tropism of HSV-1 to this mammary tumour receptor.35,36 HSV capable of targeting cells and tissues is also described in WO 03/068809.